How will Sandy compare with the D.C. derecho? Sandy is a slow-moving, large-scale storm capable of strong winds and heavy rain over an extended period. Whereas the derecho devastated locations across the D.C. area in a matter of minutes. Impact-wise, we’d recommend preparing for the same impacts as the derecho - downed trees and the potential for multiple days without power.

On the positive side, utilities and local governments and the public will have had several days warning this time versus several hours (if that) for the derecho. So, there’s a chance the impacts may not be quite as bad or last quite as long. But you should prepare for the worst and hope for the best.

When will we see the worst of the storm? When will it begin and end? There may be some showers Sunday and Sunday night (especially east of I-95) and likely increasing winds, but the strongest winds and heaviest rain should occur from midday Monday into or through the day Tuesday. Wind and rain should ease Tuesday night into Wednesday.

What are the worst- and best-case scenarios? As of now, we are probably looking at either a “very bad” scenario (70% chance) or a “worst-case” scenario (30% chance). See our storm scenario post for a full explanation.

How strong will the winds be and what is the risk of power outages? A High Wind Watch has been issued for Sunday night through Tuesday. Right now we think peak winds late Monday into early Tuesday will be around 25-45 mph sustained west of I-95 with gusts to 45-60 mph, and 30-50 mph sustained east of I-95 with gusts to 50-65 mph. In a worst-case scenario, winds could be more like 30-45 mph sustained west of I-95 with gusts to 45-60 mph, and 35-55 mph sustained east of I-95 with gusts to 55-70 mph. Either way, power outages are likely and many could be without power for multiple days.

How much rain will we get? What about flooding? A Flood Watch and Coastal Flood Watch are in effect. Right now we’re estimating rainfall totals of 3-6” west of I-95 and 4-8” east of I-95, resulting in flooding of low-lying areas and rivers/streams/creeks. In a worst-case scenario, rainfall could be more like 4-7” west of I-95 and 5-10” east of I-95, resulting in more severe flooding. In either case, these are average estimates and locally higher or lower totals are always possible.

How will this compare to Hurricane Isabel in 2003? The winds may be similar in strength. But Sandy’s likely track just to our northeast is such that we shouldn’t see a storm surge up the Potomac like we did with Isabel, which passed to our southwest. Surge and winds are always worst just to the north and east of where the storm tracks.

Simulation of blocking flow in atmosphere (Penn State)

Isn’t the storm not supposed to be a hurricane once it gets here? Technically, yes. The storm is losing its tropical characteristics as it moves north into cooler air. But, it’s expected to strengthen again as an unusual pattern featuring multiple jet streaks (rivers of fast-moving winds high up in the atmosphere) pumps the storm with a new shot of energy.

What will schools and governments (federal and local) do on Monday-Tuesday? Seems likely there will be a lot of closings.

What’s the weather for the Marine Corps Marathon? Not too bad, all things considered. Should be just breezy and cloudy with a 30% chance of showers and temperatures in the 50s.

One model suggests a little wet snow could fall west of town if enough cold air wraps in. ( StormVistaWxModels )

What about the Maryland and Delaware shore? Major coastal flooding is possible for Maryland and Delaware beaches. In the worst-case scenario, severe to historic coastal flooding is possible with 48 hours or more of onshore flow. The combination of a storm surge of 5 feet and astronomically high tides could raise water levels 10 feet above normal.

How does this compare to other historic storms? Many have compared Sandy to the Perfect Storm of 1991, and experts have warned it may be even worse. Generally speaking, though, each storm has its own character and sometimes you just can’t compare until all is said and done.

Dan Stillman is a meteorologist and editor for the Capital Weather Gang. He earned an M.S. in Meteorology from Texas A&M University, and a B.S. in Atmospheric, Oceanic and Space Sciences from the University of Michigan.

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